Metastable aluminum-alloyed δ-plutonium shrinks rapidly and pure α-plutonium swells rapidly at 4 K. At ambient temperature alloyed δ-plutonium swells about 10−3 as fast as it shrinks at 4 K, but its bulk density decreases more slowly than would be inferred from the increase in its lattice parameter determined by X-ray diffraction. These results might be explained as the result of ingrowth of the opposite phases, but they have not been found in X-ray diffraction. The cryogenic results may be explained by ingrowth of an amorphous phase with density intermediate between those of α and δ. This phase, when formed from alloyed δ-plutonium, rapidly but not instantaneously anneals to δ-plutonium at temperatures ⪆100K; when formed from pure α-plutonium it anneals to α at similar temperatures. The room temperature discrepancy between growth of lattice parameter and length of δ-plutonium is also explained by ingrowth of a denser amorphous phase that is continuously formed and annealed out.
@article{arxiv.2604.23491,
title = {Amorphous High Density Plutonium},
author = {J. K. Katz and A. Rollett and R. J. Hemley},
journal= {arXiv preprint arXiv:2604.23491},
year = {2026}
}